4.2.3 Land suitability for forest plantation and natural vegetation

The following land suitability assessment was performed for those species actually present inside the study area: Quercus suber, Tetraclinis articulata, Pinus halepensis, and Eucaliptus camaldulensis.

The methodology used is based on the F.A.O’s guide "Land evaluation for forestry" (1984). The land suitability, whose target is to find better and more suitable places for the implementation of forestry species, analyses soil conditions and climatic factors.

The present work was carried out with collected fieldwork data regarding exclusively the land characteristics; economic and social aspects were taken into account just as a broad background to the evaluation, but no data were collected for a cost-benefit analysis. The study area, in fact, plays a relevant role from an economic and social point of view. Being close to the two most important Moroccan towns, Rabat and Casablanca, the Ben Slimane forest is considered a favourable tourism destination; in addition all the study area is used for animal grazing and cork harvesting by the local inhabitants. For these reasons the species considered for land suitability are those, among the natural and artificial forest species, that allow a sustainable economy for tourism, pasture, and the production of fuel and paper wood.

Methodology

The steps used by F.A.O methodology are the following:

• Selection of the species;
• Determination of ecological parameters that enable to understand the species behaviour regarding edaphic and environmental conditions;
• Determination of classes for each ecological parameter;
• Collection of geomorphologic (slope) and soil data (drainage, pH, depth, texture, organic carbon, calcium carbonate, and coarse fragment) data;
• Data processing.

Climatic data such as elevation range, mean annual temperature, and mean annual rainfall were not considered because of the homogeneity of the study area from this point of view.

Each class of every ecological parameter was classified with a determined value for each species. The value ranges from zero (worst conditions) to one (optimum conditions). This was done to show the behaviour of the species regarding the chosen parameters as shown in the Table 42 - Table 48.

Each of this values was then multiplied for a weight, an integer number ranging from 1 to 5, to obtain a value reflecting the importance of every ecological parameter for each species. For example the weight for the limiting factors, such as drainage for Tetraclinis articulata, was set as 1 while the parameters to which the species was indifferent were set at 5.

To enhance the limiting factors, the values of each relevé, whose weight was different from one, have been summarised (å ¹_nx) and divided by the sum of the their weights (å ¹_nw), and then multiplied by the product of the values whose the weight was equal to 1 (Y).

In this way we obtained a number from 0 to 1 for each relevé, that reflects the importance of the limiting factor, giving it more weight because it is multiplied to the final value.

This number was used to give an index to every land unit according to the most representative facet in the relevé. This index was then converted to a suitability class, as shown in Table 50.

Results and discussion

Figure 34 - Figure 37 show the land suitability classification for the considered species.

Suitabilities for afforestation, production and naturalistic conservation were overlapped as layers, to show the potential use of the considered species for economic purposes (paper wood, fuel wood, cork, and grazing in forest) and for a naturalistic conservation of the forest, related to its historic importance and to recreational purposes. Eucalyptus camaldulensis has been considered a more importance economic aim becouse it is most appreciated as fuel and for the production of paper. Pinus halepensis is the next in importance, used for timber wood, and then Quercus suber, for grazing under forest and cork harvesting, and Tetraclinis articulata, whose timber is used for making tools. This procedure does not take into account the economic factor of the use (Figure 38).

From a naturalistic point of view, we considered the four species in this order of importance: Quercus suber and Tetraclinis articulata, because they are the original species of this area, and then Pinus halepensis and Eucalyptus camaldulensis, which are species introduced with plantations (Figure 39). A land suitability for soil conservation was performed for units showing a strong erosion risk (classes 3 to 5) and considering Tetraclinis articulata and Pinus halepensis more suitable for soil conservation, as they are more rustic than Eucalyptus camaldulensis and Quercus suber (Figure 40). Results are shown in Table 51.

Analysing the results for land suitability in the four land systems (plateau of Ben Slimane, western scarp of the plateau, eastern scarp of the plateau, and southern reliefs), it can be noticed that the species utilised for the afforestation, Pinus halepensis and Eucaliptus camaldulensis (Figure 36 and Figure 37), show high suitability values for most of the surface of the study area. This result arises from the particular ecology of these species, which adapt quite well even to difficult environmental conditions and soils.

Ii is evident when analysing the results for each species in the four land systems, that in the plateau of Ben Slimane, Quercus suber is suitable from moderate to high undulating planation surface while it is marginally suitable in the flat areas where the soil drainage condition, one of the limiting factor in addition to slope steepness, is poor and the soil texture is clayey. Moreover in the undulating surface the few areas marginally suitable for oak are on calcareous substratum. Along the eastern scarp of the plateau, oak is found to be marginally or not suitable at all because of the high slope steepness. The same results for the western scarp of plateau were found but this is mainly due to poor soil drainage. In the southern reliefs, cork oak is moderately suitable where the soil is well drained and the slopes are not very steep (Figure 34). This potential distribution of Quercus suber is very similar to its actual distribution, except for the undulating surface covered by agricultural crops. Taking these results into account, it is possible to suppose that in the past, cork oak forests did not accupy a much greater area.

The Quercus suber, Tetraclinis articulata is marginally suitable in most of the Ben Slimane plateau, except in some areas where local soil conditions allow an increase of suitability values. This species presents high and moderate suitability in the eastern scarp of the plateau and in the southern reliefs where good soil drainage and the high steepness of the slopes are favourable factors for its diffusion. Along the western scarp of the plateau bad soil drainage is probably the main cause of the Tetraclinis absence; in this area only few hills with calcareous substratum result highly suitable (Figure 35).

It can be noticed that also in this case the potential distribution of Tetraclinis articulata, according only to soil and climatic conditions, is very similar to the actual distribution even if some of these areas were afforested with Pinus halepensis.

As previously stated, Pinus halepensis is from moderately to highly suitable in the western scarp of the plateau, in the southern reliefs, and in the Ben Slimane plateau, but it is not suitable in the flat areas and in the western scarp of the plateau mainly because of the poor soil drainage (Figure 36).

Eucaliptus camaldulensis results marginally suitable along the eastern and western scarp of the plateau and in the southern reliefs where the high steepness becomes a limiting factor for its diffusion (Figure 37). On the other hand the high adaptability of this species to grow on poor drained soils, permits it to be used for wood production in the dayas. In this report Eucalyptus gomphocephala, even if present as afforestation, was not be used taken into account as its behaviour is very similar to that of the Eucalyptus camaldulensis.

According to the map suitability of the selected species, potential maps for wood production, natural forestry, and soil conservation were set up.

As for wood production, Eucaliptus camaldulensis results highly is in the area of the southern reliefs and in the plateau of Ben Slimane while Pinus halepensis is highly suitable along the eastern scarp and in the areas of the plateau rich of calcareous soils.

For the natural conservation target, Quercus suber and Tetraclinis articulata, being autochtonous in the study area, were chosen as principal species. Thus it arises that these two species are highly or moderately suitable in the southern reliefs, in the eastern scarp of plateau, and in the undulating area of the plateau of Ben Slimane. In this scenario, the Eucaliptus, where the oak and Tetraclinis are not suitable or marginally suitable, can occupy the flat area and the dayas, while the calcareous soils in the plateau are more suitable for Pinus halepensis.

For the soil conservation, taking into account the areas with an erosion risk class bigger than 2, it is evident that the highly risky areas could be protected through afforestation of Tetraclinis artculata and Pinus halepensis for their considerable suitability to steep slopes while Eucaliptus camaldulensis could be used in areas with a lower risk class with lower steepness.